This study is to assess the extent to which physical fitness, achieved through long-term systematic exercise, maintains mental processing speed in older men. In particular, the alm is to determine if the benefits of physical fitness do not begin to manifest themselves in the older men until a certain age, or if fitness is the critical variable, irrespective of the age of these sen. Young men and older men who are: nonexercisers, aerobic exercisers (e.g., runners), and racket sportsmen will be tested. The young men will range in age from 20-35 and the older men will be from threes age ranges; 60-64, 65-69, and 70+. Subjects will be screened medically and will be given an exercise stress test to determine their level of physical fitness (indexed by V02 max). individuals will be accepted into the study on the basis of fitness level. Older nonexercisers whose fitness level is low and older exercisers whose fitness level is high will be invited to participate in a series of speeded mental processing tasks that age graded in complexity: sample, disjunctive, and choice reactions. In each of these tasks, the subject will be presented the word LEFT or RIGHT in a block of trials and must press a left or right button in response to these words. The word will be embedded in a 4 row by 6 column matrix. The ease with which it can be discriminated will be varied by surrounding it with a signs, with letter's chosen randomly from A-, or from A-. In the simple reaction, the subject will be instructed to treat the matrix as a flash of light and to press only one button (in a block of trials) as quickly as possible. In the disjunctive reaction, the subject will respond to one of the words (e.g., LEFT) with the appropriate button press (e.g., left) and will refrain from responding to the other word. In the choice reaction, the subject will make either a left or a right button press to the target word in this task, the subject will be required to vary the direction in which he responds to LEFT or RlGHT. On some trials he must press the same button the word designates and on other trials he must press the opposite button. Subjects will be given one practice session and three test sessions. Mental processing speed will be measured using reaction time and the latency of the P300 component of the event-related brain potential. In addition to these measures of processing speed, subjects will be administered the Wechsler Adult Intelligence Scale, the Buschke Selective Reminding test, and a memory-for-designs test.